1. Technical Field of the Invention
The present invention relates to an apparatus for generating a tera-Herz wave, and a tuning method of the apparatus.
2. Description of the Related Art
A region of a far-infrared radiation or sub-millimeter wave having a frequency range of 1 to 3 THz is positioned in a light wave-radio wave interface and so its field has been left undeveloped both in technology and application in contrast to the light wave and the radio wave, which have been developed in their own fields. This field of far-infrared radiation or sub-millimeter wave, however, has been more and more important in effective utilization of a frequency band (1 to 3 THz) in wireless communications, accommodation of ultra-high communications, environmental measurement by use of imaging or tomography utilizing properties of an electromagnetic wave in such a frequency band, and application to biology and medicine. Hereinafter, a far-infrared radiation and a sub-millimeter wave in the frequency band (1 to 3 THz) is called a “THz wave”.
FIG. 1A is an illustration showing a principle for generating the THz wave. In the figure, reference numeral 1 indicates nonlinear optical crystal (e.g., LiNbO3), 2 indicates a pump light (e.g., YAG laser beam), 3 indicates an idler light, and 4 indicates a THz wave.
When the pump light 2 is incident upon the nonlinear optical crystal 1 having Raman and far-infrared activities in a constant direction, an Stimulated Raman Scattering effect (or parametric interaction) generates the idler light 3 and THz wave 4 through an elementary excitation wave (polariton) of a material. In this case, an energy conservation law represented by Equation (a) and momentum conservation law (phase matching condition) represented by Equation (b) are established among the pump light 2 (ωp), THz wave 4 (ωT), and idler light 3 (ωi). It is to be noted that Equation (b) represents a vector relationship and a non-collinear phase matching condition can be represented as shown in the upper right of FIG. 1A.ωp=ωT+ωi  (a)κp=κT+κi  (b)
The idler light 3 and THz wave 4 generated at this time have a spatial spread and their wavelengths change continuously in accordance with their outgoing angles. The generation of the broad idler light and THz wave in this single-path arrangement is called THz-wave parametric generation (TPG).
It is to be noted that a basic optical parametric process is defined as annihilation of one pump photon and simultaneous generation of one idler photon and one signal photon. When the idler or signal light resonates and if the intensity of the pump light exceeds a constant threshold, parametric oscillation occurs. Moreover, the annihilation of one pump photon and simultaneous generation of one idler photon and one polariton are combined to constitute Stimulated Raman Scattering scattering, which is included in parametric interaction in a broad sense.
As described above, in FIG. 1A, the pump light 2 including a Z-axis polarized light and having a frequency ωp is incident upon the LiNbO3 crystal which is the nonlinear optical crystal 1. Then, by parametric wavelength conversion, the idler light 3 (frequency ωi) having a frequency slightly lower than that of the pump light 2, and the THz wave 4 (frequency ωT) whose frequency equals to a difference of the frequency between the pump and idler lights are generated. Moreover, outgoing directions of the idler light and THz-wave beam are given by the non-collinear phase matching condition (angles θ, φ), and have angles which slightly differ with each wavelength.
However, when only the pump light 2 is injected into the crystal 1, the idler light 3 and THz wave 4 are spontaneously emitted lights generated from parametric noises, and a spectrum line width therefore reaches several hundreds of GHz and is remarkably broad. Moreover, the generated THz wave is very faint and has a problem that its major part is absorbed in the nonlinear optical crystal when it goes through it by several hundreds of micrometers.
FIG. 1B is a principle diagram of an injection-seeded THz-wave parametric generator (is-TPG) which solves the problem (Japanese Patent Application Laid-Open No. 2002-72269). In this is-TPG, a seed light 5 whose frequency is lower than the pump light 2 by 1 to 3 THz and whose spectrum line width is narrow is injected as a seed of idler light generation into the LiNbO3 crystal 1. Thereby, the spectrum line width of the THz wave, which corresponds to the difference frequency between the pump light 2 and idler light 3, is narrowed.
FIG. 2 is a configuration diagram of the related-art is-TPG based on the above-described principle. In this diagram, a light source of the pump light 2 is an Nd:YAG laser which has a fixed wavelength and single frequency, and a light source of the seed light 5 is a semiconductor laser whose wavelength is variable. The seed light 5 is reflected by reflection mirrors M1, M2 and injected into the MgO:LiNbO3 crystal 1 with a slight angle (θIN) to the pump light 2.
According to the method and apparatus of FIGS. 1B and 2, a second laser device is used to inject the seed light 5 in a generation direction of the idler light 3 generated by the pump light. Therefore, a more intense idler light can be generated as compared in the generation of the idler light 3 in the nonlinear optical crystal only by the spontaneous emission. Thereby, it has been confirmed that the light intensity of the idler light 3 of this direction and the intensity of the THz wave 4 satisfying the non-collinear phase matching condition is also greatly enhanced.
Moreover, directivity of the idler light 3 is high, and the laser beam is used in both the pump light 2 and seed light 5. Therefore, not only it has similarly been confirmed that the directivity of the generation direction of the generated THz wave 4 is enhanced but also the spectrum width can also greatly be narrowed.
However, for the tuning of the THz-wave frequency, since an incidence angle θIN of the seed light 5 satisfies the non-collinear phase matching, the incidence angle θIN has to be adjusted in accordance with the variable wavelength of the seed light 5.
For example, in the above-described THz-wave generation apparatus, when the THz wave is greatly changed, for example, in a frequency range of 1 to 2.5 THz, a y-axis stage and mirror M1 need to be manually finely adjusted so as to allow the seed light beam to intersect with the pump light beam at an optimum angle (e.g., 1 to 2.5°) in a point A on a LiNbO3 crystal incidence plane. Therefore, the related-art THz-wave parametric generator requires time and labor in tuning the frequency, and there is a problem that it is difficult to incorporate the unit into spectrometer.
It is to be noted that an allowable range of the seed light incidence angle is about ±0.16°. Therefore, when a narrow width of about 300 GHz or less is adjusted, the incidence angle adjustment is not necessary, and it is possible to tune the THz wave only by the adjustment of the seed light frequency.
Moreover, FIG. 3 is a principle diagram of the THz-wave generation apparatus in which a beam deflection element and confocal optical system are used (Japanese Patent Application No. 2001-187735, not laid open). This beam deflection element 6 can control the incidence angle of the pump light at a high speed. When the same beam deflection element and confocal optical system are applied to the seed light, the incidence angle of the seed light beam can be adjusted with one mirror. However, even when the beam deflection element is used, in order to maintain the injection-seeding, it is necessary to constantly monitor the seed light wavelength and to control a laser beam scanner outgoing angle. There is a disadvantage that the system becomes complicated.